A train is comprised of an engine car of inertia \(M\), and 3 more cars of inertias \(m_{1}, m_{2}\), and \(m_{3}\), respectively. The acceleration of the train is \(a\). Friction can be ignored.

(a) What is the net force acting on the train?

(b) What is the force in the coupler pulling the last car \(\left(m_{3}\right)\) ?

(c) What is the force in the coupler between the engine and the first car \(\left(m_{1}\right)\) ?